Inertially tracked objects

ABSTRACT

Described are computer-based methods and apparatuses, including computer program products, for inertially tracked objects with a kinematic coupling. A tracked pose of a first inertial measurement unit (IMU) is determined, wherein the first IMU is mounted to a first object. The tracked pose of the first IMU is reset while the first object is in a first reproducible reference pose with a second object.

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application is a continuation of U.S. application No.12/821,365, entitled “INERTIALLY TRACKED OBJECTS,” filed Jun. 23,2010,which is hereby incorporated by reference herein in its entirety

FIELD OF THE INVENTION

The present invention relates generally to computer-based methods andapparatuses, including computer program products, for inertially trackedobjects.

BACKGROUND

Orthopedic joint replacement surgery may involve arthroplasty of a knee,hip, or other joint (e.g., shoulder, elbow, wrist, ankle, finger, etc.).During joint replacement surgery, a surgeon typically removes diseasedbone from the joint and replaces the resected bone with prostheticimplant components. Challenges of joint replacement surgery includedetermining the appropriate position for implant components within thejoint relative to the bone and other implant components and accuratelycutting and reshaping bone to precisely correspond to the plannedplacement of the implant components. Inaccurate positioning of implantsmay compromise joint performance and reduce implant life.

A surgical system for joint replacement surgery can include a hapticdevice configured to be manipulated by a surgeon to guide a surgicalcutting tool to perform a procedure on a patient. For example, a surgeoncan manipulate the haptic device to sculpt a bone so that an implantcomponent can be installed on the sculpted bone. Prior to surgery, athree dimensional model of the bone is created using softwaretechniques. The software model is used to generate a surgical plan, thatincludes, for example, resecting bone (e.g., using the surgical cuttingtool) and inserting implant components. During surgery, the surgeonmanipulates the haptic device to move the surgical tool to cut bone, andthe haptic device provides force feedback to prevent the surgeon frommoving the surgical tool in a way that does not conform with thesurgical plan. For example, if the surgeon's movement of the hapticdevice would cause the surgical tool to resect too much of the patient'sbone, the haptic device can apply resistance against the surgeon'smovement to prevent the resection. A navigation or tracking system canbe used to determine a pose (i.e., position and/or orientation) of thebone, the haptic device, the surgical tool, and/or other objects ofinterest. As is well known, pose data from the tracking system can beused for registration and real-time object tracking.

For example, U.S. patent application Ser. No. 11/357,197 (U.S. Pub. No.2006/0142657), which is hereby incorporated by reference herein in itsentirety, describes that objects in physical space (e.g., anatomy,surgical tools, etc.) may be registered to any suitable coordinatesystem, such as a coordinate system being used by a process running on acomputer associated with a surgical system. For example, utilizingobject pose data captured by a tracking system, the surgical system canassociate the physical anatomy and the surgical tool with arepresentation of the anatomy (such as a computer-generatedthree-dimensional model or image of the anatomy). Based on the trackedobject and registration data, the surgical system can determine, forexample, (a) a spatial relationship between the image of the anatomy andthe relevant physical anatomy and (b) a spatial relationship between therelevant physical anatomy and the surgical tool so that the computingsystem can superimpose (and continually update) a virtual representationof the tool on the image of the anatomy, where the relationship betweenthe virtual representation of the tool and the image of the anatomy issubstantially identical to the relationship between the actual surgicaltool and the physical anatomy. Additionally, by tracking not only thetool but also the relevant anatomy, the surgical system can compensatefor movement of the relevant anatomy during the surgical procedure(e.g., by adjusting a virtual object that defines a surgical cuttingboundary in response to the detected movement of the anatomy).

The tracking system enables the surgical system to determine (or track)in real-time a pose of tracked objects, such as the bone. One commontype of tracking system is an optical tracking system that includes anoptical camera configured to locate in a predefined coordinate spacespecially recognizable markers (e.g., LEDs or reflective spheres) thatare attached to the tracked object. However, optical tracking systemsrequire a direct line of sight between the optical camera and themarkers. This restricts the movement of the surgeon during surgerybecause the surgeon cannot interfere with the optical communicationbetween the optical camera and the markers. As a result, the surgeon'smovement is limited not only by the location of physical equipment inthe operating room but also by lines of sight between the optical cameraand markers. Further, other unavoidable surgical side-effects caninterfere with the optical communication, such as bone debris that isgenerated during a bone resection and occludes the surface of one ormore markers. Additionally, while optical tracking systems are oftenaccurate, they can be cost-prohibitive.

Another type of tracking system is an inertial tracking system, whichuses an inertial measurement unit (IMU) to track an object. An IMU is anelectronic device that includes a combination of accelerometers and/orgyroscopes to measure characteristics of an object, such as the object'svelocity, orientation, and/or gravitational forces. For example, an IMUcan measure three degrees of freedom of the acceleration and threedegrees of freedom of the angular rate of the IMU. Using thesemeasurements, the inertial tracking system can estimate the current sixdegree of freedom pose of the IMU based on a previously determined IMUpose (e.g., an initial (or starting) reference pose) and changes inacceleration and angular rate of the IMU over time. While inertialtracking systems can be more cost-effective than other tracking systems,in general inertial tracking systems introduce greater error throughdrift. That is, since new poses are calculated from previouslydetermined poses and measured changes in acceleration and angular rate(without reference to any external references), the errors of thetracking process are cumulative such that the error in each newestimated IMU pose grows with time. Specifically, the inertial trackingsystem integrates the linear accelerations and angular velocitiesprovided by the IMU to calculate the new IMU pose (the acceleration datais often double integrated). The accumulated error leads to “drift,” oran ever-increasing difference between where the inertial tracking systemthinks the IMU is located and the actual IMU pose. If the drift is notcompensated for, the pose of the tracked object (e.g., the bone) can beincorrectly predicted based on the difference between the predicted IMUpose and the actual IMU pose. This could cause the surgical system to beimproperly configured unbeknownst to the surgeon, which could lead tothe surgical plan being carried out improperly (e.g., performingimproper resections).

In view of the foregoing, a need exists for methods and devices whichcan overcome the aforementioned problems so as to enable computerassisted surgery (CAS) to be carried out when drift occurs between anIMU's calculated pose and actual pose by resetting a reference pose ofthe IMU.

SUMMARY OF THE INVENTION

The techniques described herein provide surgical systems and methods forinertially tracking objects and eliminating drift associated with thetracked objects by resetting the reference pose of the inertialtracker(s) mounted to the tracked objects. A reproducible reference poseis established between the tracked objects such that the tracked objectscan be repeatedly and accurately placed into the reproducible referencepose to reset a tracking calculation between the objects. When the driftassociated with an inertial tracker mounted to one (or more) of thetracked objects becomes too large, the tracked objects can be placedinto the reproducible reference pose to reset the reference pose of theinertial tracker(s). Advantageously, a cost-effective inertial trackingsystem can be deployed that not only eliminates drift but also avoidsdisadvantages associated with other tracking systems. For example,optical tracking systems are often more expensive than inertial trackingsystems and the surgeon's movement is often restricted to maintain aline of sight between the markers and the optical camera.

In one aspect, there is a method of resetting a tracking element. Themethod includes determining, by an inertial tracking system, a trackedpose of a first inertial measurement unit (IMU), wherein the first IMUis mounted to a first object. The method includes resetting, by theinertial tracking system, the tracked pose of the first IMU while thefirst object is in a first reproducible reference pose with a secondobject.

In another aspect, there is a method for inertially tracking one or moreobjects. The method includes determining, by an inertial trackingsystem, that an error associated with a first IMU, a second IMU, orboth, exceeds a predetermined threshold, wherein the error is indicativeof a first discrepancy between a tracked pose of the first IMU and anactual pose of the first IMU, a second discrepancy between a trackedpose of the second IMU and an actual pose of the second IMU, or both.The method includes resetting the tracked pose of the first IMU, thetracked pose of the second IMU, or both, based on a reproduciblereference pose, wherein the reproducible reference pose includes apredetermined transformation indicative of a first pose of the secondIMU with respect to a first pose of the first IMU, including setting thetracked pose of the first IMU to the first pose of the first IMU, orsetting the tracked pose of the second IMU to the first pose of thesecond IMU, or both.

In a further aspect, there is a computer program product. The computerprogram product is tangibly embodied in a computer readable storagemedium. The computer program product includes instructions beingoperable to cause a data processing apparatus to determine a trackedpose of a first inertial measurement unit (IMU), wherein the first IMUis mounted to a first object. The computer program product includesinstructions being operable to cause a data processing apparatus toreset the tracked pose of the first IMU while the first object is in afirst reproducible reference pose with a second object.

In another aspect, there is an apparatus for inertially tracking one ormore objects. The apparatus includes an inertial tracking systemconfigured to determine a tracked pose of a first inertial measurementunit (IMU), wherein the first IMU is mounted to a first object. Theinertial tracking system is configured to reset the tracked pose of thefirst IMU while the first object is in a first reproducible referencepose with a second object.

In a further aspect, there is an apparatus for inertially tracking oneor more objects. The apparatus includes a means for determining atracked pose of a first inertial measurement unit (IMU), wherein thefirst IMU is mounted to a first object and resetting the tracked pose ofthe first IMU while the first object is in a first reproduciblereference pose with a second object.

In other examples, any of the aspects above can include one or more ofthe following features. The first IMU can include a first couplingconfigured to couple to a second coupling mounted to the second objectto achieve the first reproducible reference pose.

In some examples, the inertial tracking system determines a tracked poseof a second IMU, wherein the second IMU is mounted to the second object.The inertial tracking system can reset the tracked pose of the secondIMU while the first object is in the first reproducible reference posewith the second object. Resetting the tracked pose of the first IMU andresetting the tracked pose of the second IMU can include determining thefirst object and the second object are in the first reproduciblereference pose, and resetting the tracked pose of at least one of thefirst IMU and the second IMU based on the first reproducible referencepose.

In other examples, the first reproducible reference pose includes apredetermined transformation indicative of a first pose of the secondIMU with respect to a first pose of the first IMU. A tracked pose of thesecond IMU can be determined with respect to the first IMU based on thetracked pose of the first IMU, the tracked pose of the second IMU, andthe first reproducible reference pose. The first IMU can include a firstkinematic coupling and the second IMU includes a second kinematiccoupling. The first kinematic coupling can be configured tokinematically couple to the second kinematic coupling to achieve thefirst reproducible reference pose. The tracked pose of the first IMU,the tracked pose of the second IMU, or both, can be reset when thesecond kinematic coupling is kinematically coupled to the firstkinematic coupling.

In some examples, the inertial tracking system determines that an errorassociated with the first IMU, the second IMU, or both, exceeds apredetermined threshold. The tracked pose of the first IMU, the trackedpose of the second IMU, or both, are reset based on the firstreproducible reference pose. The first reproducible reference pose caninclude a predetermined transformation indicative of a first pose of thesecond IMU with respect to a first pose of the first IMU, and resettingthe tracked pose of the first IMU and resetting the tracked pose of thesecond IMU can include setting the tracked pose of the first IMU to thefirst pose of the first IMU, or setting the tracked pose of the secondIMU to the first pose of the second IMU, or both.

In other examples, a third object is configured to couple to at leastone of the first object and the second object to achieve a secondreproducible reference pose, data is received indicative of the firstobject being coupled to at least one of the second object in the firstreproducible reference pose and the third object in the secondreproducible reference pose, and a tracked pose of the second IMU, athird IMU mounted to the third object, or both is reset based on thedata. A third object can be configured to couple to at least one of thefirst object and the second object to achieve a second reproduciblereference pose, data can be received indicative of the third objectbeing coupled to the at least one of the first object and the secondobject in the second reproducible reference pose, and a tracked pose ofa third IMU mounted to the third object can be reset based on the data.

In some examples, the second object includes a first portion configuredto couple to the first IMU to achieve the first reproducible referencepose and a second portion configured to couple to a second IMU mountedto a third object to achieve a second reproducible reference pose. Theinertial tracking system can determine a tracked pose of the second IMU.The intertial tracking system can reset the tracked pose of the firstIMU, the tracked pose of the second IMU, or both, when the first IMU iscoupled to the first portion of the second object in the firstreproducible reference pose and the second IMU is coupled to the secondportion of the second object in the second reproducible reference pose.The inertial tracking system can determine that an error associated withthe first IMU exceeds a predetermined threshold, and can reset thetracked pose of the first IMU based on the first reproducible referencepose.

In other examples, resetting includes determining the first object andthe second object are in the first reproducible reference pose andresetting the tracked pose of the first IMU based on the firstreproducible reference pose. Determining the first object and the secondobject are in the first reproducible reference pose can includedetermining the second object is kinematically coupled to the firstobject. Data can be received indicative of the first IMU beingkinematically coupled to the second IMU to achieve the reproduciblereference pose. A tracked pose of the second IMU can be determined withrespect to the first IMU based on the tracked pose of the first IMU, thetracked pose of the second IMU, and the reproducible reference pose.

The techniques, which include both methods and apparatuses, describedherein can provide one or more of the following advantages. An inertialtracking system can be used that reduces or eliminates drift thatcommonly occurs with IMUs by resetting the reference position of theIMUs based on a reproducible reference pose. Reducing or eliminatingdrift allows the inertial tracking system to more accurately track IMUs.The inertial tracking system can be more cost effective and simpler thanother tracking systems. For example, inertial tracking systems do notneed to include expensive optical devices or multi-part systems. IMUs(e.g., wireless IMUs) can be easily mounted to a patient's anatomy(e.g., a bone) or surgical instruments (e.g., a haptic device or aprobe) without creating line-of-sight barriers. IMUs can be mounted inthe same incision made for bone resections, which can eliminateadditional incisions required to mount trackers to the patient's anatomy(e.g., bone pins) and can improve patient recovery time. IMUs can belightweight and small enough to minimize any forces transmitted to thepatient compared to other trackers (e.g., compared to optical trackingarrays).

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating the principles of theinvention by way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the presentinvention, as well as the invention itself, will be more fullyunderstood from the following description of various embodiments, whenread together with the accompanying drawings.

FIG. 1 illustrates an exemplary surgical computer system;

FIG. 2 illustrates an exemplary diagram of poses of inertially trackedobjects and drifts associated with the inertially tracked objects.

FIG. 3 illustrates an exemplary diagram of a reproducible reference posefor two inertially tracked objects.

FIG. 4 illustrates an exemplary computer implemented method forresetting an inertial tracking element;

FIGS. 5A-5C illustrate an exemplary embodiment of a kinematic couplingfor establishing a reproducible reference pose;

FIG. 6 illustrates an exemplary computer implemented method forresetting the reference pose for multiple inertially tracked objects;

FIG. 7 illustrates an exemplary diagram of an instrumented linkage forresetting inertially tracked objects;

FIG. 8 illustrates an exemplary computer implemented method forresetting inertially tracked objects with an instrumented linkage;

FIGS. 9A-9B illustrate exemplary diagrams of a first inertially trackedobject being used to reset a second inertially tracked object, a thirdinertially tracked object, or both; and

FIG. 10 illustrates an exemplary computer implemented method for using afirst inertially tracked object to reset a second inertially trackedobject, a third inertially tracked object, or both.

DETAILED DESCRIPTION

Presently preferred embodiments of the invention are illustrated in thedrawings. Although this specification refers primarily to objecttracking during image-guided orthopedic surgical procedures involvingthe knee joint, it should be understood that the subject matterdescribed herein is applicable to other types of navigated surgicalprocedures, including imageless surgical procedures, as well as tonon-surgical applications involving object tracking.

According to the present invention, a reference position for an inertialtracking element or tracker (e.g., for an IMU) can be reset based on areproducible reference pose to eliminate an error (e.g., drift)associated with the inertial tracker. The reproducible reference posecan be achieved by kinematically coupling the inertial tracker to asecond object, such as a second inertial tracker or an untrackedcoupling element. The inertial tracking system can subsequently trackthe inertial tracker based on the reset reference position. By utilizingthe reproducible reference pose of the present technology, navigatedsurgical procedures can be accurately and efficiently performed throughthe manipulation of a surgical tool, both with or without hapticguidance.

FIG. 1 shows an embodiment of an exemplary surgical computer system 10in which the inertially tracked objects and techniques described hereincan be implemented. A similar exemplary system is described in detail,for example, in U.S. Patent Application Publication No. 2006/0142657,published Jun. 29, 2006, which is hereby incorporated by referenceherein in its entirety. In a preferred embodiment, the surgical computersystem is the RIO® Robotic Arm Interactive Orthopedic System,manufactured by MAKO Surgical Corp., Fort Lauderdale, Fla. The surgicalsystem 10 includes a computing system 20, a haptic device 30, and atracking system (which is described in further detail herein). Inoperation, the surgical system 10 enables comprehensive surgicalplanning and provides haptic guidance to a surgeon as the surgeonperforms a surgical procedure. Although included for completeness in theillustrated embodiment, the haptic device 30 and its associated hardwareand software are not necessary to perform the techniques describedherein.

The computing system 20 includes hardware and software for operation andcontrol of the surgical system 10. Such hardware and/or software isconfigured to enable the surgical system 10 to perform the techniquesdescribed herein. In FIG. 1, the computing system 20 includes a computer21, a display device 23, and an input device 25. The computing system 20may also include a cart 29.

The computer 21 may be any known computing system but is preferably aprogrammable, processor-based system. For example, the computer 21 mayinclude a microprocessor, a hard drive, random access memory (RAM), readonly memory (ROM), input/output (I/O) circuitry, and any otherwell-known computer component. The computer 21 is preferably adapted foruse with various types of storage devices (persistent and removable),such as, for example, a portable drive, magnetic storage (e.g., a floppydisk), solid state storage (e.g., a flash memory card), optical storage(e.g., a compact disc or CD), and/or network/Internet storage. Thecomputer 21 may comprise one or more computers, including, for example,a personal computer (e.g., an IBM-PC compatible computer) or aworkstation (e.g., a SUN or Silicon Graphics workstation) operatingunder a Windows, MS-DOS, UNIX, or other suitable operating system andpreferably includes a graphical user interface (GUI).

The display device 23 is a visual interface between the computing system20 and the user. The display device 23 is connected to the computer 21and may be any device suitable for displaying text, images, graphics,and/or other visual output. For example, the display device 23 mayinclude a standard display screen (e.g., LCD, CRT, plasma, etc.), atouch screen, a wearable display (e.g., eyewear such as glasses orgoggles), a projection display, a head-mounted display, a holographicdisplay, and/or any other visual output device. The display device 23may be disposed on or near the computer 21 (e.g., on the cart 29 asshown in FIG. 1) or may be remote from the computer 21 (e.g., mounted ona wall of an operating room or other location suitable for viewing bythe user). The display device 23 is preferably adjustable so that theuser can position/reposition the display device 23 as needed during asurgical procedure. For example, the display device 23 may be disposedon an adjustable arm (not shown) that is connected to the cart 29 or toany other location well-suited for ease of viewing by the user. Thedisplay device 23 may be used to display any information useful for amedical procedure, such as, for example, images of anatomy generatedfrom an image data set obtained using conventional imaging techniques,graphical models (e.g., CAD models of implants, instruments, anatomy,etc.), graphical representations of a tracked object (e.g., anatomy,tools, implants, etc.), digital or video images, registrationinformation, calibration information, patient data, user data,measurement data, software menus, selection buttons, status information,and the like.

The input device 25 of the computing system 20 enables the user tocommunicate with the surgical system 10. The input device 25 isconnected to the computer 21 and may include any device enabling a userto provide input to a computer. For example, the input device 25 can bea known input device, such as a keyboard, a mouse, a trackball, a touchscreen, a touch pad, voice recognition hardware, dials, switches,buttons, a trackable probe, a foot pedal, a remote control device, ascanner, a camera, a microphone, and/or a joystick.

The computing system 20 is in communication with a computing device 31of the haptic device 30 and anatomy trackers 43 a and 43 b, generally43. The communication interface may be any known interface such as, forexample, a wired interface (e.g., serial, USB, Ethernet, CAN bus, and/orother cable communication interface) and/or a wireless interface (e.g.,wireless Ethernet, wireless serial, infrared, and/or other wirelesscommunication system) and may include a software interface resident onthe computer 21 and/or the computer 31. In some embodiments, computer 21and 31 are the same computing device.

The surgical system 10 also includes a tracking system, comprising oneor more tracking elements (e.g., the anatomy trackers 43). A trackingelement is configured to be attached to a tracked object and isregistered to the tracked object. Data from the tracking element is used(e.g., by the computer 21 and/or the computer 31) to determine a pose(i.e., position and/or orientation) of the tracked object (i.e., atracked pose). Tracked objects may include, for example, anatomy, thehaptic device 30, surgical tools, and/or other objects of interest. Insome embodiments, the tracking system may include a computing device(e.g., a separately located computing device or software and/or hardwarethat is integrated into the computer 21, the computer 31, and/or thetracking element) that receives data from the tracking element anddetermines a pose of the tracked object with respect to a coordinateframe of interest, such as a coordinate frame of the tracking system. Asis well known, the determined pose can be transformed into othercoordinate frames of reference, such as a coordinate frame of a virtualenvironment of the surgical system 10. Data from the tracking system canbe used, for example, to register tracked objects (e.g., to register thepatient's bone to an image of the bone) and to track movement of objectsduring a surgical procedure.

For example, using pose data from the tracking system, the surgicalsystem 10 is able to register (or map or associate) coordinates in onespace to those in another to achieve spatial alignment or correspondence(e.g., using a coordinate transformation process as is well known).Objects in physical space may be registered to any suitable coordinatesystem, such as a coordinate system being used by a process running onthe computer 21 and/or the computer 31. For example, utilizing pose datafrom the tracking system, the surgical system 10 is able to associatethe physical anatomy (i.e., physical space) with a representation of theanatomy (such as an image displayed on the display device 23) (i.e.,image space). Based on tracked object and registration data, thesurgical system 10 may determine, for example, a spatial relationshipbetween the image of the anatomy and the relevant physical anatomy(i.e., between the image space and the physical space). Knowing thisrelationship, the image of the anatomy (e.g., displayed on the displaydevice 23) can be made to move in correspondence with the movement ofthe relevant tracked physical anatomy. The surgical system 10 may alsodetermine, for example, a spatial relationship between the relevantphysical anatomy and a surgical tool (not shown) so that the computingsystem 20 can superimpose (and continually update) a virtualrepresentation of the surgical tool on the image of the anatomy, wherethe relationship between the virtual representation of the surgical tooland the image of the anatomy is substantially identical to therelationship between the physical surgical tool and the actual physicalanatomy. Additionally, by tracking not only the surgical tool but alsothe relevant anatomy, the surgical system can compensate for movement ofthe relevant anatomy during the surgical procedure (e.g., by adjusting avirtual object that defines an anatomical cutting boundary in responseto the detected movement of the physical anatomy).

Registration may include any known registration technique, such as, forexample, image-to-image registration (e.g., monomodal registration whereimages of the same type or modality, such as fluoroscopic images or MRimages, are registered and/or multimodal registration where images ofdifferent types or modalities, such as MRI and CT, are registered);image-to-physical space registration (e.g., image-to-patientregistration where a digital data set of a patient's anatomy obtained byconventional imaging techniques is registered with the patient's actualanatomy); and/or combined image-to-image and image-to-physical-spaceregistration (e.g., registration of preoperative CT and MRI images to anintraoperative scene). One example of a process for registering patientanatomy to an image of the anatomy is described in U.S. PatentApplication Pub. No. 2006/0142657, published Jun. 29, 2006, which ishereby incorporated by reference herein in its entirety. The surgicalsystem 10 may also include a coordinate transform process for mapping(or transforming) coordinates in one space to those in another toachieve spatial alignment or correspondence. For example, the surgicalsystem 10 may use the coordinate transform process to map positions oftracked objects (e.g., patient anatomy, etc.) into a coordinate systemused by a process running on the computer 31 and/or the computer 21. Asis well known, the coordinate transform process may include any suitabletransformation technique, such as, for example, rigid-bodytransformation, non-rigid transformation, affine transformation, and thelike.

In addition to registration, the tracking system enables the surgicalsystem 10 to determine (or track), in real-time, poses of trackedobjects. According to an embodiment of the present invention, as shownin FIG. 1, the tracking system includes an inertial tracking system thatcomprises at least one tracking element (or inertial tracker) configuredto be disposed on (or incorporated into) a tracked object and acomputing device (e.g., the computer 21, the computer 31, and/or acomputing device incorporated into the tracking element) for determininga pose (i.e., a tracked pose) of the tracked object. The tracked pose iscalculated based on data from the tracking element and a registrationbetween the tracked object and the tracking element. In an exemplaryembodiment, the tracking element includes a first inertial tracker (theanatomy tracker 43 a) and a second inertial tracker (the anatomy tracker43 b). The anatomy trackers 43 a and 43 b are configured to be affixedto the tracked objects (i.e., the femur F and the tibia T, respectively)in a secure and stable manner and each includes an IMU having a knowngeometric relationship to the respective tracked object. The knowngeometric relationship can be determined, for example, using aconventional registration process. The IMU can be, for example, anInertia-Link® IMU and Vertical Gyro provided by MicroStrain, Inc.,Williston, Vt., a NavChip™ provided by InterSense, Inc., Billerica,Mass., and/or the like. In operation, the IMU measures changes in itsacceleration and angular rate over time. This information, combined withan initial (or starting or previous) reference pose of the IMU and theIMU's known geometric relationship to the tracked object, enable thesurgical system 10 to calculate a current pose of the tracked objectbased on the tracked pose of the IMU.

In FIG. 1, the anatomy tracker 43 is disposed on a relevant portion of apatient's anatomy (such as a bone). The anatomy tracker 43 includes afixation device for attachment to the anatomy. The fixation device maybe, for example, a bone pin, surgical staple, screw, clamp, wearabledevice, intramedullary rod, or the like. In some embodiments, theanatomy tracker 43 is configured for use during knee replacement surgeryto track the femur F and the tibia T of the patient. In the embodimentof FIG. 1, the anatomy tracker 43 includes a first tracker 43 a adaptedto be disposed on the femur F and a second tracker 43 b adapted to bedisposed on the tibia T. When installed on the patient and registered tothe respective portion of the anatomy, the first and second trackers 43a and 43 b enable the tracking system to determine tracked poses of thefemur F and the tibia T in real-time. While embodiments have describedthe tracking system as being an inertial tracking system, the trackingsystem may additionally include any other type of tracking system (e.g.,optical, mechanical, electromagnetic, fiber optic, and the like).

FIG. 2 illustrates an exemplary diagram 200 of poses of inertiallytracked objects and drifts associated with the inertially trackedobjects. The diagram 200 includes a first inertial tracker, a first IMU202, mounted and registered to a first object, a probe 204 (aninertially tracked probe). The actual IMU pose 206 is the actual pose ofthe first IMU 202 and the probe 204 in three-dimensional physical space(e.g., in a preoperative/intraoperative scene, such as an operatingroom). The tracked IMU pose 208 is the corresponding tracked pose of thefirst IMU 202 (and the tracked pose of the probe 204). For purposes ofillustration, the tracked IMU pose 208 is shown in the same coordinateframe of reference as the actual IMU pose 206 but could be transformedinto any coordinate frame of interest, such as, for example,three-dimensional image space, as is well known. As a result of theaccumulated error of the IMU 202, there is a drift 230 between theactual IMU pose 206 and the tracked IMU pose 208. The diagram 200includes a second inertial tracker, a second IMU 210, mounted andregistered to a second object, a bone 212. The actual IMU pose 214 isthe actual pose of the second IMU 210 in the three-dimensional physicalspace, and the tracked IMU pose 216 is the corresponding tracked pose ofthe second IMU 210 (and the tracked pose of the bone 212). There is adrift 218 between the actual IMU pose 214 and the tracked IMU pose 216.

According to an embodiment of the present invention, tracking errorcaused by drift (e.g., the drift 230 or the drift 218) can be mitigatedby using a reproducible reference pose. FIG. 3 illustrates an exemplarydiagram of a reproducible reference pose 300 for two inertially trackedobjects. In the reproducible reference pose, the inertially trackedobjects have a known geometric relationship relative to one another(determined, for example, using registration and/or calibrationprocesses). The reproducible reference pose can be established betweenthe tracked objects such that the tracked objects can be repeatedly andaccurately placed into the reproducible reference pose to reset atracking calculation between the objects. For example, prior to trackingthe relative position of the tracked objects, the tracked objects can betemporarily coupled together in the reproducible reference pose toestablish an initial (or starting) reference pose for the trackedobjects. The tracked objects can then be decoupled and tracked by theinertial tracking system. When the drift associated with the inertialtracker of one (or both) tracked objects becomes too large, the trackedobjects can be placed back into the reproducible reference pose to reset(i.e., unbias or “zero”) the pose of one inertial tracker relative tothe other to thereby eliminate the drift error. The reproduciblereference pose can be achieved, for example, by temporarilykinematically coupling the inertial trackers of the tracked objectstogether. The inertial tracking system can subsequently track theinertial trackers based on this reset reference position. In theembodiment of FIG. 3, the reproducible reference pose 300 includes thefirst IMU 202 removably coupled to the second IMU 210. The first IMU 202(and the probe 204) and the second IMU 210 (and the bone 212) can beplaced into the reproducible reference pose as indicated by arrow 220 inFIG. 2. The first IMU 202 and the second IMU 210 can include, forexample, kinematic couplings as described with reference to FIGS. 5A-5Cbelow. As described with reference to FIG. 2, the first IMU 202 ismounted and registered to the probe 204, and the second IMU 206 ismounted and registered to the bone 212. Because the coupling between thefirst and second IMUs 202, 210 is kinematic, the exact pose of the firstIMU 202 with respect to the second IMU 210 (or vice versa) is known(e.g., via a predetermined transformation) when the IMUs 202, 210 areplaced into the reproducible reference pose. Similarly, from theregistration process, a predetermined transformation is known betweenthe first IMU 202 and the probe 204 and between the second IMU 210 andthe bone 212 such that the exact pose of the probe 204 with respect tothe bone 212 can be determined.

The predetermined transformations can be determined using conventionalcalibration, registration, and/or coordinate transformation processes.For example, the first IMU 202 includes a first IMU coordinate frame222, and the second IMU 210 includes a second IMU coordinate frame 224(e.g., the coordinate frames of inertial sensors of the IMUs). Thediagram 200 includes a reference frame 226, which can be any coordinateframe of interest, such as the coordinate system of the inertialtracking system, the coordinate system of the image space, or anothercoordinate system of interest. As is well known, transformations can becalculated between the objects and the IMUs (e.g., between the bone 212and the second IMU 210 as described below with reference to FIG. 4) suchthat the coordinate frame of the object can be mapped to the coordinateframe of the IMU (or vice versa), which can then be transformed into thereference frame 226.

Although the IMUs 202, 210 are described in connection with a probe anda bone, in other embodiments, IMUs can be mounted to other objects(e.g., other portions of anatomy, a surgical tool, a cutting jig, anoperating table, a floor, the haptic device 30, etc.). Additionally, thelength of the drifts 230 and 218 can be larger or smaller. For example,the drift can become larger over time (e.g., the drift can effectivelybe zero when the inertial tracking system begins tracking the IMUs andincrease over time). The tracked IMU poses 208 and 216 are shown asexemplary three dimensional shifts from the actual IMU poses 206 and214. The tracked IMU poses 208 and 216 can drift in any position and/ororientation from the actual IMU poses 206 and 214 (e.g., the tracked IMUpose 208 can be above the actual IMU pose 206, below the actual IMU pose206, etc.).

In one embodiment, multiple IMUs can be affixed to a single object andused to track the object. For example, two or more IMUs can be affixedto a bone or other tracked object. One advantage of using multiple IMUsto track a single object is that pose data from the multiple IMUs can beaveraged to minimize drift error. In one embodiment, there are multipleIMUs on a single object (e.g., a bone) but only one coupling (e.g., akinematic coupling). The coupling can be affixed to one of the IMUs ormounted separately to the object. A relationship between the coordinatesystems of the multiple IMUs and/or between the IMUs and the couplingcan be established via registration. To reset all of the IMUs, anothertracked object (e.g., a tracked probe) can be coupled to the coupling asdescribed herein. In particular, once the pose of any IMU on the objectis registered to the pose of the coupling (or to another IMU that isregistered to the coupling), resetting of that IMU can be accomplishedby coupling the tracked object to the coupling, provided the IMUs andthe coupling are affixed to the object in a stable manner and do notmove relative to each other or the object. Advantageously, in thisembodiment, the pose data from the multiple IMUs can be averaged tominimize drift.

According to an embodiment, multiple IMUs (e.g., two IMUs) can be usedto track multiple objects (e.g., two objects) where one IMU is affixedto each object. For example, as described above in connection with FIGS.2 and 3, both the IMU 202 and the IMU 210 can be used to simultaneouslytrack the probe 204 and the bone 212, respectively. The IMUs 202, 210can be coupled together to create the reproducible reference pose 300,and the poses for both the IMU 202 and the IMU 210 can be reset based onthe reproducible reference pose. Using two IMUs to track two objects(where an IMU is mounted to each object) and resetting the referenceposes of the IMUs when the two objects are placed in a reproduciblereference pose is further described with reference to FIG. 6.

In another embodiment, one IMU can be used to track a first object whilea second object is untracked. The second object may be, for example, abone that is fixed in place thereby eliminating the need to track thebone. In this embodiment, the reproducible reference pose is similar tothe reproducible reference pose 300 of FIG. 3 except the IMU 210 isreplaced with an untracked coupling element (e.g., a kinematic coupling)that is mounted to the bone 212 and that does not include an IMU.Alternatively, the IMU 210 can be temporarily (or permanently)deactivated. To reset the IMU 202, the IMU 202 (attached to the probe204) and the untracked coupling element (attached to the bone 212) areplaced into a reproducible reference pose (e.g., the reproduciblereference pose 300) as described above in connection with FIG. 3. Usingan IMU to track one object and resetting the IMU's reference pose whenthe first object is coupled to a second untracked object in areproducible reference pose is further described with reference to FIG.4.

FIG. 4 illustrates an exemplary computer implemented method 400 forresetting an inertial tracking element (e.g., an IMU). This embodimentdescribes using one inertial tracker to track a first object (e.g., asurgical instrument comprising an IMU), where a first IMU is mounted toa first object, while a second object is untracked. Preferably, prior tostep 402, the first and second objects are placed in a firstreproducible reference pose to establish an initial (or starting)reference pose of the first IMU. At step 402, the inertial trackingsystem determines a tracked pose of the first IMU. At step 404, theinertial tracking system determines whether an error associated with thefirst IMU exceeds a predetermined threshold. For instance, in oneembodiment, for surgical navigation, the predetermined threshold can beapproximately 0.3 mm and 0.3 degrees. If the error does not exceed thepredetermined threshold, the method 400 proceeds back to step 402. Ifthe error exceeds the predetermined threshold, the method 400 proceedsto step 406. At step 406, the inertial tracking system determines thefirst object and the second object are in the first reproduciblereference pose. At step 408, the inertial tracking system resets thetracked pose of the first IMU while the first object is in the firstreproducible reference pose with the second object.

Referring to step 402 and FIG. 2, for example, the inertial trackingsystem (e.g., the inertial tracking system described above withreference to FIG. 1) determines the tracked IMU pose 208 of the firstIMU 202. The first IMU 202 can calculate various measurements indicativeof the pose of the first IMU 202 (e.g., angular rate, angular position,Euler angles, rotation matrices, linear acceleration, and linearvelocity). The measurements can be used (e.g., by the first IMU 202, theinertial tracking system, etc.) to estimate a six degree of freedom(6DOF) pose of the first IMU 202. For example, linear accelerationvectors and angular rate vectors can be integrated to estimate the poseof the first IMU 202.

In some embodiments, IMU measurements are transformed to a referencecoordinate frame (e.g., the inertial tracking system's coordinateframe). For example, the inertial tracking system can transform thefirst IMU 202 measurements, which are calculated in the first IMUcoordinate frame 222, to the reference frame 226. In some examples, therotation between the reference frame 226 and the first IMU coordinateframe 222 can be calculated based on infinitesimal rotation matrices(e.g., differential rotations) and/or integrated using knowndifferential equations.

When an IMU is attached to an object (e.g., a rigid body), the estimated6DOF pose of the IMU can be used to calculate a 6DOF pose of the object.Referring to FIG. 2, the first IMU 202 is mounted to the probe 204. Atransformation between the pose of the first IMU 202 and the pose of theprobe 204 can be determined, thereby allowing the inertial trackingsystem to calculate a tracked pose of the probe 204 based on the trackedpose of the first IMU 202 (e.g., based on a known geometry between theIMU 202 and the probe 204 determined using conventional calibrationand/or registration processes). For example, a predeterminedtransformation can be established between the origin of the IMU 202 andthe tip of the probe 204. Advantageously, the inertial tracking systemcan determine the tracked IMU pose 208 and use the predeterminedtransformation to determine the tracked pose of the probe 204.

Some objects may not have a known geometry with respect to the IMU. Forexample, the second IMU 210 can be intra-operatively mounted to thepatient's bone 212, such that the geometric relationship between thesecond IMU 210 and the bone 212 is unknown. When the second IMU 210 ismounted to the bone 212, the transformation between the second IMU 210and the bone 212 can be determined by registering the bone 212. Forexample, the IMU 202 attached to the probe 204 (e.g., a sharp probe) canbe used to collect positions (points) on the surface of the bone 212.The inertial tracking system can use the collected positions to fit thebone 212 to a preoperatively obtained three dimensional model of thebone (e.g., obtained from a CT scan, MRI scan, etc.). For example, thecollected points can be fit to the three dimensional model of the bone212 to register the bone 212 (e.g., in a least-squares manner). In someembodiments, the first IMU 202 and the second IMU 210 can be placed intoa reproducible reference pose (e.g., as described with reference to FIG.3) to reset a reference position (a starting position) for the first IMU202 and/or the second IMU 210 before collecting the positions on thebone 212. In some examples, other tools and/or methods can be used tocalculate the transformation between the second IMU 210 and the bone212. For example, a multi-degree of freedom robotic arm can be used tocollect positions on the surface of the bone 212. This is furtherdescribed with reference to FIGS. 7-8. As described with reference toFIG. 1, registration may include any known registration technique, suchas, for example, image-to-image registration, image-to-physical spaceregistration, and/or combined image-to-image and image-to-physical-spaceregistration.

Referring to step 404, over time an IMU accumulates error or drift,resulting in a disassociation between the actual IMU pose and thetracked IMU pose (e.g., the drift 230 between the actual IMU pose 206and the tracked IMU pose 208). The inertial tracking system can beconfigured to estimate when the error associated with an IMU (the drift)exceeds a certain threshold, requiring the reference pose of the IMU tobe reset. For example, the inertial tracking system can be configured toalert a user every ten minutes, fifteen minutes, or other time intervalthat the drift has grown too large (e.g., based on a theoretical rate ofdrift, the elapsed amount of time since the last reset, etc.). Anotherway to trigger when to reset the reference pose is to use a checkpoint(e.g., a mechanical divot) as described, for example, in U.S. PatentPub. No. US 2008/0004633, published Jan. 3, 2008, and herebyincorporated by reference herein in its entirety. For example, acheckpoint can be affixed to a bone and then coupled with an IMU toconfirm that the distance between the IMU origin and the checkpoint hasnot shifted.

Although some embodiments of the present application describe the use ofa predetermined threshold for determining when to reset an IMU, an IMUcan be reset at any time by placing the IMU into the reproduciblereference pose whether or not the error is greater or less than apredetermined threshold. For example, method 400 can omit step 404 andproceed directly to step 406 whenever the surgeon desires, such as twoor three times during a surgical procedure, after an elapsed period oftime (e.g., ten minutes), and/or whenever the surgeon thinks a reset maybe appropriate. Similarly, method 600 (shown in FIG. 6) can omit step606 and proceed directly to the step 608.

Referring to step 406, the inertial tracking system determines the firstand second objects are in the first reproducible reference pose (e.g., areproducible reference pose like that shown in FIG. 3). For example,assume the untracked second object is a bone (e.g., the bone 212) thatis fixed in place and includes an untracked coupling element mounted tothe bone (e.g., a coupling element similar to that shown as IMU 210 inFIGS. 2 and 3 except the untracked coupling element does not include anIMU or the IMU has been deactivated). The first object, for example, asurgical instrument (e.g., the first IMU 202 and probe 204), can betracked with respect to the untracked bone using only one IMU (e.g., thefirst IMU 202). The reproducible reference pose can be achieved bycoupling the first IMU to the untracked coupling element mounted to thebone.

The inertial tracking system can determine the first object and thesecond object are in the first reproducible reference pose based on acoupling between the first object and a second object. For example, thefirst IMU can include a first coupling configured to couple to a secondcoupling mounted to the second object to achieve the first reproduciblereference pose. In some embodiments, the first IMU is configured tokinematically couple to the first object. FIGS. 5A-5C illustrate anexemplary embodiment of a kinematic coupling 500 for establishing areproducible reference pose. Further, for example, U.S. patentapplication Ser. No. 12/479,510 (U.S. Pub. No. 2009/0306499), which ishereby incorporated by reference herein in its entirety, describes akinematic coupling for a self-detecting kinematic clamp assembly. FIGS.5A-5C illustrate different views of the kinematic coupling 500 and thusthe following description is generally applicable and refers to any ofthe views. While FIGS. 5A-5C show an embodiment of a kinematic coupling,one skilled in the art can appreciate that the kinematic coupling can beany type of repeatable kinematic mount.

The kinematic coupling 500 includes a first coupling 502 and a secondcoupling 504 that are adapted to be repeatedly, accurately, andremovably, coupled together to achieve a reproducible reference pose,such as the reproducible reference pose 300 shown in FIG. 3. In someembodiments, the first and/or second couplings 502, 504 can be disposedon an inertial tracker (e.g., on an IMU) and/or on an element or part(or connection portion) that does not include an inertial tracker. Insome embodiments, the first and/or second couplings 502, 504 can includean IMU (e.g., the couplings can comprise an inertial tracker). The firstand/or second couplings 502, 504 can be adapted to connect to an object.For example, the first and/or second couplings 502, 504 can be removablysecured to an object (e.g., a bone) or permanently secured to an object(e.g., a surgical instrument). The first and second couplings 502, 504are preferably configured to be removably engaged together in a specificgeometric configuration and to resist engagement in other geometricconfigurations. For example, in one embodiment, the first coupling 502comprises magnets 506A, 506B and 506C, collectively magnets 506, andgrooves (e.g., v-grooves) 508A, 508B and 508C, collectively grooves 508.The second coupling 504 comprises magnets 510A, 510B and 510C,collectively magnets 510, and balls 512A, 512B and 512C, collectivelyballs 512. In some embodiments, the balls 512 are steel balls.

The magnets 506 of the first coupling 502 and the magnets 510 of thesecond coupling 504 are positioned so that when the engagement surface502A of the first coupling 502 is brought into close proximity with theengagement surface 504A of the second coupling 504, each of the magnets506 is aligned over a corresponding magnet 510 (e.g., the magnet 506A issubstantially aligned with the magnet 510A, the magnet 506B issubstantially aligned with the magnet 510B, and the magnet 506C issubstantially aligned with the magnet 510C). Preferably, the magnets506, 510 are oriented to generate an attraction force only when thefirst and second couplings 502, 504 are arranged in a particularrelative geometric configuration. For example, the magnets 506, 510 areoriented such that the poles of the magnets 506 located on theengagement surface 502A have the opposite polarity of the poles of thecorresponding magnets 510 located on the engagement surface 504A so thatwhen the magnets 506, 510 are properly aligned and placed in closeproximity (e.g., the north pole (N) of the magnet 506A is placed inclose proximity to the south pole (S) of the magnet 510A, the north pole(N) of the magnet 506B is placed in close proximity to the south pole(S) of the magnet 510B, and the south pole (S) of the magnet 506C isplaced in close proximity to the north pole (N) of the magnet 510C) theopposite poles attract and the first coupling 502 becomes kinematicallyassembled (e.g., repeatably, accurately, and removably connected) to thesecond coupling 504. The magnets 506 and 510 enable quick assembly anddisassembly of the first coupling 502 from the second coupling 504 andalso prevent assembly of the kinematic coupling 500 in an incorrectconfiguration. As can be seen in FIG. 5A, the magnets 506, 510 arearranged such that the magnets 506, 510 would resist or opposemisalignment of the couplings 502, 504. For example, if the secondcoupling 504 is rotated such that the magnet 510A is aligned over themagnet 506C, the south poles (S) of the magnets 510A and 506C wouldrepel one another thereby causing the second coupling 504 to repel oroppose the first coupling 502. Advantageously, the magnets 510 and 506can be configured such that each of the magnets 510 is attracted to acorresponding magnet 506 only when the first coupling 502 and the secondcoupling 504 are in the proper relative orientation.

The grooves 508 and the balls 512 are arranged symmetrically around acentral axis 514, 516 of each coupling 502, 504. When the engagementsurface 502A of the first coupling 502 is brought into close proximityto the engagement surface 504A of the second coupling 504 in the propergeometric configuration, the balls 512 are oriented over the grooves 508so that each ball 512 sits in a corresponding groove 508 (e.g., the ball512A sits in the groove 508A). Advantageously, in some embodiments thealignment of the balls 512 and the grooves 508 ensures that each timethe second coupling 504 is disconnected, and then re-connected to thefirst coupling 502, the second coupling 504 returns to exactly the sameposition. Thus, a specific assembly configuration of the first coupling502 to the second coupling 504 can be easily preserved regardless of thenumber of times the second coupling 504 is connected to or removed fromthe first coupling 502. In one embodiment, the balls 512 should be ofsufficient size so that when the first coupling 502 is kinematicallyassembled to the second coupling 504, there is a slight gap between theengagement surface 502A of the first coupling 502 and the engagementsurface 504A of the second coupling 504.

FIGS. 5A-5C show three balls 512 and three corresponding grooves 508,which result in a true kinematic coupling because the six degrees offreedom of the kinematic coupling 500 are constrained by six points ofcontact. However, other configurations can also be used to removablyconnect the first coupling 502 to the second coupling 504 withoutdeparting from the principles described herein. For example, the grooves508 can be located on the second coupling 504 and the balls 512 can belocated on the first coupling 502. In alternative embodiments, featuresother than balls and grooves can be employed, as described, for example,in U.S. patent application Ser. No. 12/644,964, filed Dec. 22, 2009, andincorporated by reference herein in its entirety. In some examples, thesecond coupling 504 can be removably connected to the first coupling 502using a Maxwell Mount, a Kelvin Mount, a Canoe Ball/Vee Groove Mount, aThree Tooth Coupling, a semi-kinematic mount, or any other type ofremovable, repeatable connection. In some embodiments, if departing froma true kinematic coupling, care should be taken to not over-constrainthe design, which can result in increased wear and tear on thecomponents (e.g., the grooves 508 of the first coupling 502 and theballs 512 of the second coupling 504). Additionally, the kinematiccoupling 500 can include any number of magnets or no magnets.

Advantageously, because the kinematic coupling 500 allows the secondcoupling 504 to be repeatedly and accurately attached to and removedfrom the first coupling 502, the first coupling 502 and the secondcoupling 504 can be repeatedly connected to achieve the reproduciblereference pose. For example, the IMU 202 can be configured to includethe first coupling 502, and a bone can include a connector configured toinclude the second coupling 504 (the connector need not include an IMU).The IMU 202 can be kinematically coupled to the connector to achieve areproducible reference pose similar to, for example, the reference pose300 in FIG. 3.

The kinematic coupling 500 can include a detection mechanism. Thedetection mechanism can include a state indicator that indicates acoupling of the first and second couplings 502, 504 of the kinematiccoupling 500 based on the detection mechanism. In some embodiments, thedetection mechanism can include a circuit configured to monitor contactpoints between the grooves 508 and balls 512. In some embodiments, thedetection mechanism can include load sensors (e.g., on the sides of thegrooves 508) configured to measure load from the balls 512. The inertialtracking system can use the value of the state indicator to determinewhen the first and second objects (e.g., when kinematic couplingsmounted to the first and second objects) are properly coupled to achievethe reproducible reference pose.

Referring to step 408, the inertial tracking system resets the trackedpose of the first IMU while the first object is in the firstreproducible reference pose with the second object. In one embodiment,an optical or magnetic proximity sensor, or electrical circuit, can beused to automatically reset the inertial sensor of the first IMU oncethe two mating kinematic couplings are connected. Alternatively, thesurgical system 10 can include a manual reset switch or software buttonactivated by the surgeon. When the kinematic couplings are connected,the reference pose of the first IMU is reset (e.g., the tracked IMU poseis set to equal the reference pose), such that the pose of the first IMUrelative to the second object can be determined based on knowntransformations between the first IMU and the untracked coupling elementand between the untracked coupling element the second object (e.g., thebone 212). For example, a predetermined transformation can be calculatedfrom the coordinate system for an IMU on a surgical instrument to acoordinate system of the bone. The exact pose of the IMU to the bone isknown when the surgical instrument and the bone are placed in thereproducible reference pose (e.g., via kinematic couplings on thesurgical instrument and the bone). For example, the transformation fromthe coordinate system of the IMU to the coordinate system of theuntracked coupling element mounted to the bone can be predetermined.Further, a transformation from the coordinate system of the untrackedcoupling element to the bone can be determined via, for example, aregistration process. Once the first IMU is reset, each new pose(tracked IMU pose) can be calculated as an offset from the IMUreference, or starting, position. Throughout a surgical procedure, thefirst and second objects can be reconnected in the first reproduciblereference pose as often as necessary (e.g., based on step 404) to resetthe first IMU reference position (and to consequently zero-out thedrift).

While method 400 of FIG. 4 describes using one inertial tracker to trackan object (e.g., a surgical instrument comprising an IMU), more than oneinertial tracker can be used to track multiple objects. FIG. 6illustrates an exemplary computer implemented method 600 for resettingthe reference position of multiple tracked objects, where a first IMU ismounted to a first object and a second IMU is mounted to a secondobject. Preferably, prior to step 602, the first and second objects areplaced in a first reproducible reference pose to establish an initial(or starting) reference pose of the first and second IMUs. At step 602,the inertial tracking system determines a tracked pose of the first IMU.At step 604, the inertial tracking system determines a tracked pose ofthe second IMU. At step 606, the inertial tracking system determineswhether an error associated with the first IMU, the second IMU, or bothexceeds a predetermined threshold. If neither error is greater than thepredetermined threshold, the method 600 proceeds back to step 602. Ifone or more of the errors is greater than the predetermined threshold,the method 600 proceeds to step 608. At step 608, the inertial trackingsystem determines the first object is in a first reproducible referencepose with the second object. At step 610, the inertial tracking systemresets the tracked pose of the first IMU while the first object is inthe first reproducible reference pose with the second object. At step612, the inertial tracking system resets the tracked pose of the secondIMU while the first object is in the first reproducible reference posewith the second object. Although some embodiments of the presentapplication describe the use of a predetermined threshold fordetermining when to reset an IMU, an IMU can be reset at any time byplacing the IMU into the reproducible reference pose regardless ofwhether the error is greater than or less than a predeterminedthreshold.

Referring to steps 602-604 and FIG. 2, the inertial tracking systemdetermines the tracked IMU pose 208 of the first IMU 202 and the trackedIMU pose 216 of the second IMU 210. Referring to step 606, the inertialtracking system determines that an error associated with the first IMU202 and/or the second IMU 210 is greater than a predetermined threshold.The error is indicative of a discrepancy between the tracked IMU pose208 and the actual IMU pose 206 (drift 230) and/or a discrepancy betweenthe tracked IMU pose 216 and the actual IMU pose 214 (drift 218).

Referring to step 608, the first IMU can include a first kinematiccoupling and the second IMU can include a second kinematic coupling. Forexample, the first IMU 202 can include the first coupling 502 of thekinematic coupling 500 and the second IMU 210 can include the secondcoupling 504 of the kinematic coupling 500. The inertial tracking systemcan receive data indicative of the first IMU 202 being kinematicallycoupled to the second IMU 210 to achieve the reproducible reference pose300 of FIG. 3. In some embodiments, the tracked IMU pose 208, thetracked IMU pose 216, or both, are reset when the kinematic coupling onthe first IMU 202 is kinematically coupled to the kinematic coupling onthe second IMU 210. Referring to steps 610 and 612, the inertialtracking system resets the tracked IMU pose 208 of the first IMU 202 andthe tracked IMU pose 216 of the second IMU 210 based on the reproduciblereference pose 300 (e.g., resets a reference pose for the IMUs). In someexamples, the inertial tracking system can reset just one of the trackedIMU poses (e.g., just the tracked IMU pose 208 or the tracked IMU pose216). For example, if only the error associated with the first IMU isover the predetermined threshold, the inertial tracking system can justreset the tracked pose of the first IMU.

The inertial tracking system can determine a tracked pose of the secondIMU with respect to the first IMU based on the tracked pose of the firstIMU, the tracked pose of the second IMU, and the first reproduciblereference pose. The first reproducible reference pose can include apredetermined transformation indicative of a first pose of the secondIMU with respect to a first pose of the first IMU. For example, thereproducible reference pose 300 can include a predeterminedtransformation indicative of a first pose (a reference pose) of thesecond IMU with respect to a first pose of the first IMU. The inertialtracking system can calculate the exact pose (e.g., position andorientation) of the first IMU 202 and the second IMU 210 based on thepredetermined transformation (e.g., when the first IMU 202 and thesecond IMU 210 are in the reproducible reference pose 300). The inertialtracking system can set the tracked IMU pose 208 of the first IMU 202 tothe first pose of the first IMU, and can set the tracked IMU pose 216 ofthe second IMU 210 to the first pose of the second IMU. The inertialtracking system can track (e.g., determine tracked poses) for the firstIMU 202 and the second IMU 210 after the first IMU 202 and the secondIMU 210 leave the reproducible reference pose 300. Advantageously, theinertial tracking system can calculate (in real time) the pose of thefirst and second IMUs with respect to each other based on thepredetermined transformation and/or the tracked poses of the IMUs.

FIG. 7 illustrates an exemplary diagram 700 of an instrumented linkage702 for resetting inertially tracked objects. The instrumented linkage702 includes links 703A, 703B, 703C, 703D and 703E (collectively links703), joints 704A, 704B, 704C and 704D (collectively joints 704), andconnection portions 706A and 706B (collectively connection portions706). The diagram 700 includes a haptic device 708 that includes arobotic arm 710 and a first IMU 712 with a first coordinate frame 714.The diagram 700 also includes a second IMU 716 with a second coordinateframe 718 mounted to a bone 720 (e.g., a tibia or femur bone). Theconnection portion 706A is configured to connect to the first IMU 712 toachieve a first reproducible reference pose. The connection portion 706Bis configured to connect to the second IMU 716 to achieve a secondreproducible reference pose. Preferably, the connection portions 706A,706B include kinematic couplings (e.g., as described above in connectionwith FIGS. 5A-5C) to couple to corresponding kinematic couplings of thefirst and second IMUs 712, 716, respectively. In this embodiment, thefirst reproducible reference pose is defined by the kinematic couplingbetween the connection portion 706A and the first IMU 712. Similarly,the second reproducible reference pose is defined by the kinematiccoupling between the connection portion 706B and the second IMU 716. Thefirst and second reproducible references poses may be the same ordifferent, depending on factors such as the calibration of the kinematiccouplings, the geometry of the objects being coupled, and the like.

In some examples, one end of the instrumented linkage 702 is mounted toan object rather than including a connection portion 706. For example,the instrumented linkage 702 can have a proximal end (the end comprisingthe connection portion 706A) affixed to the base of the haptic device708 and a freely moveable distal end (the end comprising the connectionportion 706B) such that the connection portion 706B can be repeatablycoupled to the bone 720 of the patient (e.g., via the second IMU 716).In some examples, the proximal end may be affixed to any other suitablelocation (such as, for example, to a rail of an operating table, a legholder, etc.).

Each joint 704 incorporates one or more position sensors (not shown) totrack a pose of the instrumented linkage 702. The position sensors mayinclude any suitable sensor, such as a joint encoder. In operation, asthe ends of the instrumented linkage 702 move (or are manipulated), thelinks 703 and joints 704 move accordingly. Data from the positionsensors (and appropriate software) and the known geometry of the links703 are used to determine a pose of one end of the instrumented linkage702 (e.g., the distal end) relative to the other end (e.g., the proximalend) of the instrumented linkage 702. In this manner, regardless of theactual pose of the first IMU 712 and the second IMU 716 in physicalspace, the instrumented linkage 702 can be manipulated to connect to thefirst and second IMUs 712, 716. Advantageously, the mechanical linkage702 allows objects that can not be physically manipulated into areproducible reference pose (e.g., like the reproducible reference pose300 in FIG. 3) to still be placed into a reproducible reference pose.For example, the haptic device 708 may be repositioned during aprocedure without having to be recalibrated to a bone motion trackingsystem. In some embodiments, connection portions 706 can similarlyfunction as joints 704 (e.g., including positions sensors to track apose of the instrumented linkage 702).

A predetermined transformation between the second IMU 716 and the bone720 can be determined by registering the bone 720 to a three-dimensionalmodel of the bone as described above with respect to FIG. 4. Apredetermined transformation between the robotic arm 710 (or anotherportion of the haptic device 710, such as the end effector 711) and thefirst IMU 712 can be determined. For example, the robotic arm 710 caninclude a number of links and joints similar to those of theinstrumented linkage 702 so the pose of the end effector 711 can becalculated. A predetermined transformation can be calculated between thefirst IMU 712 and the end effector 711 based on the robotic arm 710. Insome examples, the first IMU 712 is mounted to a different location ofthe haptic device 708 (e.g., to the end effector 711 of the robotic arm710).

FIG. 8 illustrates an exemplary computer implemented method 800 forresetting inertially tracked objects with an instrumented linkage (e.g.,the instrumented linkage 702 in FIG. 7). This embodiment includes afirst object (e.g., the haptic device 708), a second object (e.g., theinstrumented linkage 702), and a third object (e.g., the bone 720),where a first IMU is mounted to the first object and a second IMU ismounted to the third object. At step 802, the inertial tracking systemdetermines a tracked pose of the first IMU (e.g., the first IMU 712)that is mounted to the first object (e.g., the haptic device 708). Atstep 804, the inertial tracking system determines a tracked pose of thesecond IMU (e.g., the second IMU 716) that is mounted to the thirdobject (e.g., the bone 720). At step 806, the inertial tracking systemdetermines the first IMU is coupled to a first portion of the secondobject in a first reproducible reference pose (e.g., the first IMU 712is coupled to the connection portion 706A of the instrumented linkage702). At step 808, the inertial tracking system determines the secondIMU is coupled to a second portion of the second object in a secondreproducible reference pose (e.g., the second IMU 716 is coupled to theconnection portion 706B). At step 810, the tracked pose of the inertialtracking system resets the tracked pose of the first IMU, the trackedpose of the second IMU, or both, based on the first reproduciblereference pose and the second reproducible reference pose.

Referring to steps 806 and 808, the inertial tracking system candetermine the IMUs are coupled to the respective portions of the secondobject using kinematic couplings as described above with reference toFIGS. 5A-5C. Referring to step 810, the inertial tracking system can usesignals for the instrumented linkage 702 (e.g., joint 704 encodersignals) and the known geometry of the links 703 to determine thetransformation between the first IMU 712 and the second IMU 716. Asdescribed above in connection with step 606 of FIG. 6, the inertialtracking system can determine that an error associated with the firstIMU 712 and/or the second IMU 716 is above a predetermined threshold.

A transformation between the robotic arm 710 and the second IMU 716 canbe determined. For example, a transformation between the end effector711 and the second IMU 716 can be determined prior to the surgicalprocedure. In some embodiments, the location of the base of the roboticarm 710 and the second IMU 716 is determined using the first IMU 712.The robotic arm 710 may be used to register the patient's anatomy (e.g.,instead of the probe as described above with reference to FIGS. 2-4).For example, the user may use the robotic arm 710 to register the bone720. Registration may be accomplished, for example, by pointing a tip(e.g., a probe) of the distal end of the robotic arm 710 to anatomicallandmarks on the bone 720 and/or by touching points on (or “painting”) asurface of the bone 720 with the tip of the distal end of the roboticarm 710. As the user touches landmarks on the bone 720 and/or paints asurface of the bone 720, the surgical system (e.g., the surgical system10 of FIG. 1) acquires data from the position sensors in the robotic arm710 and determines a pose of the tip of the robotic arm 710.Simultaneously, the second IMU 716 provides data regarding motion of thebone 720 so that the surgical system can account for bone motion duringregistration. Based on the bone motion data and knowledge of theposition of the tip of the robotic arm 710, the surgical system 10 isable to register the bone to diagnostic images and/or an anatomicalmodel of the patient's anatomy (e.g., stored in the computing system20). As described above with reference to FIGS. 7-8, the robotic arm 710can be reregistered throughout a surgical procedure using theinstrumented linkage 702.

FIGS. 9A-9B illustrate exemplary diagrams 900, 950 of a first inertiallytracked object (e.g., a first IMU 904 disposed on a probe 908) beingused to reset a second inertially tracked object (e.g., a second IMU 910disposed on a bone 914), a third inertially tracked object (e.g., athird IMU 916 disposed on a bone 902 or a haptic device 952), or both.The third object in FIG. 9A is different from the third object in FIG.9B. FIG. 9A includes the bone 902 (e.g., a femur) as the third object.In contrast, FIG. 9B includes the haptic device 952 as the third object.Similar to FIG. 7, the haptic device 952 includes a robotic arm 954 andan end effector 956. With the exception of the different third objects,FIGS. 9A and 9B include the same remaining features and thus thefollowing description is generally applicable and refers to either ofthe diagrams 900 or 950. A first IMU 904 with a coordinate frame 906 ismounted to the first object, the probe 908. A second IMU 910 with acoordinate frame 912 is mounted to the second object, the bone 914(e.g., a tibia). A third IMU 916 with a coordinate frame 918 is mountedto the third object (either the bone 902 of FIG. 9A or the haptic device952 of FIG. 9B). As indicated by arrow 920, the first IMU 904 can beplaced in a first reproducible reference pose with the second IMU 910.As indicated by arrow 922, the first IMU 904 can be placed in a secondreproducible reference pose with the third IMU 916.

FIG. 10 illustrates an exemplary computer implemented method 1000 forusing a first inertially tracked object to reset a second inertiallytracked object, a third inertially tracked object, or both. In thisembodiment, at step 1002, the inertial tracking system determines atracked pose of a first IMU mounted to a first object (e.g., the firstIMU 904 mounted to the probe 908). At step 1004, the inertial trackingsystem determines a tracked pose of a second IMU mounted to a secondobject (e.g., the second IMU 910 mounted to the bone 914). At step 1006,the inertial tracking system determines a tracked pose of a third IMUmounted to a third object (e.g., the third IMU 916 mounted to the bone902 in FIG. 9A or the haptic device 952 in FIG. 9B). At step 1008, theinertial tracking system receives data indicative of the first objectbeing coupled to at least one of the second object and the third objectin a reproducible reference pose, for example, as indicated by arrows920 (a first reproducible reference pose) and 922 (a second reproduciblereference pose). At step 1010, the inertial tracking system resets thetracked pose of the second IMU, the third IMU, or both based on thedata.

Referring to step 1008, the first object can be used to register thesecond object and/or the third object. Referring to FIG. 9A, forexample, the probe 908 can be used to register the bone 902 and the bone914. A predetermined transformation between the second IMU 910 and thebone 914 can be determined by registering the bone 914 to athree-dimensional model of the bone as described above with respect toFIG. 4. Similarly, a predetermined transformation between the third IMU916 and the bone 914 can be determined by registering the bone 902 to athree-dimensional model of the bone. Advantageously, the predeterminedtransformation can map the object to the coordinate frame of the IMUmounted to the object (e.g., bone 914 to the coordinate frame 912 of thesecond IMU 910).

Further referring to step 1008, the first object can be used to resetthe tracked pose of the second object and/or the third object. Theinertial tracking system can determine an error associated with thesecond object and/or the third object exceeds a predetermined threshold.For example, referring to FIGS. 9A and 9B, the inertial tracking systemcan determine that an error associated with the second IMU 910 and/orthe third IMU 916 exceeds a predetermined threshold (e.g., similar tostep 606 of FIG. 6). The first object can be placed into the firstreproducible reference pose with the second object to reset the trackedpose of the second object and/or the first object can be placed into thesecond reproducible reference pose with the third object to reset thetracked pose of the third object. The first object can include a firstcoupling configured to couple to a second coupling mounted to the secondobject to achieve the first reproducible reference pose. For example,the first IMU 904 and the probe 908 can be placed into the firstreproducible reference pose with the bone 914 and the second IMU 910 asindicated by arrow 920 (e.g., using a kinematic coupling). Similarly,the first IMU 904 and the probe 908 can be placed into the secondreproducible reference pose with the bone 902 and the third IMU 916 asshown by arrow 922. Referring to step 1008, the inertial tracking systemcan use the data (e.g., information indicative of the first reproduciblereference pose, information indicative of the second reproduciblereference pose, and/or tracking information of the objects) to reset thetracked pose of the first IMU, the second IMU, and/or the third IMU.

Referring further to steps 1008 and 1010, the first object can be usedto locate the origin of the second object with respect to the thirdobject. For example, referring to FIG. 9B, the first IMU 904 can beplaced into a first reproducible reference pose with the second IMU 910as indicated by arrow 920. The first IMU 904 can be placed into a secondreproducible reference pose with the third IMU 916 as indicated by arrow922. The inertial tracking system can receive data indicative of thefirst and second reproducible reference poses and use the data todetermine the pose of the haptic device 952 with respect to the bone914. For example, the inertial tracking system can use the data tocalculate an origin of the base of the robotic arm 954 with respect tothe bone 914. As another example, alternative tracking technologies canbe used to estimate the transformation between the robotic arm 954 withrespect to the bone 914. Such tracking systems can include optical,mechanical, fiber optic, and/or electromagnetic tracking systems (e.g.,the instrumented linkage described with respect to FIGS. 7 and 8).

Another embodiment is similar to the embodiment of FIG. 9A describedabove except the second object is a second probe instead of the bone914. For example, the first object (e.g., the probe 908) could be ablunt probe and the second object (e.g., the second probe (not shown))could be a sharp probe. Either the blunt probe and/or the sharp probecould couple with the third object (e.g., the bone 902) to achieve areproducible reference pose to reset the third IMU (e.g., the third IMU916). In this embodiment, an exemplary computer implemented method forusing a first inertially tracked object and/or a second inertiallytracked object to reset a third inertially tracked object includes thefollowing steps. The inertial tracking system determines a tracked poseof a first IMU mounted to a first object (e.g., the first IMU 904mounted to the probe 908), a tracked pose of a second IMU mounted to asecond object (e.g., the second IMU 910 mounted to the second probe),and a tracked pose of a third IMU mounted to a third object (e.g., thethird IMU 916 mounted to the bone 902). The first and second objects areconfigured to couple in a first reproducible reference pose, and thethird object is configured to couple to the first object and/or thesecond object in a second reproducible reference pose (e.g., usingkinematic couplings as described in connection with FIGS. 5A-5C). Theinertial tracking system receives data indicative of the third objectbeing coupled to at least one of the first object and the second objectin the second reproducible reference pose and resets the tracked pose ofthe third IMU mounted to the third object based on the data.

While embodiments have described the tracking system as being aninertial tracking system, the tracking system may additionally includeother non-mechanical and/or mechanical tracking systems. Thenon-mechanical tracking system may include, for example, an optical (orvisual), electromagnetic, radio, or acoustic tracking system, as is wellknown. The mechanical tracking system may include, for example, a fiberoptic tracking system or an articulated arm having joint encoders) asdescribed, for example, in U.S. Patent Application Pub. No.2009/0314925, published Dec. 29, 2009, and U.S. Pat. No. 6,322,567,respectively, each of which is hereby incorporated by reference hereinin its entirety. In one embodiment, the tracking system includes amechanical tracking system having a jointed mechanical arm (e.g., anarticulated arm having six or more degrees of freedom) adapted to tracka bone of the patient.

The above-described systems and methods can be implemented in digitalelectronic circuitry, in computer hardware, firmware, and/or software.The implementation can be as a computer program product (i.e., acomputer program tangibly embodied in an information carrier). Theimplementation can, for example, be in a machine-readable storagedevice, for execution by, or to control the operation of, dataprocessing apparatus. The implementation can, for example, be aprogrammable processor, a computer, and/or multiple computers.

A computer program can be written in any form of programming language,including compiled and/or interpreted languages, and the computerprogram can be deployed in any form, including as a stand-alone programor as a subroutine, element, and/or other unit suitable for use in acomputing environment. A computer program can be deployed to be executedon one computer or on multiple computers at one site.

Method steps can be performed by one or more programmable processorsexecuting a computer program to perform functions of the invention byoperating on input data and generating output. Method steps can also beperformed by and an apparatus can be implemented as special purposelogic circuitry. The circuitry can, for example, be a FPGA (fieldprogrammable gate array) and/or an ASIC (application-specific integratedcircuit). Modules, subroutines, and software agents can refer toportions of the computer program, the processor, the special circuitry,software, and/or hardware that implements that functionality.

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor receives instructions and data from a read-only memory or arandom access memory or both. The essential elements of a computer are aprocessor for executing instructions and one or more memory devices forstoring instructions and data. Generally, a computer can include, can beoperatively coupled to receive data from and/or transfer data to one ormore mass storage devices for storing data (e.g., magnetic,magneto-optical disks, or optical disks).

Data transmission and instructions can also occur over a communicationsnetwork. Information carriers suitable for embodying computer programinstructions and data include all forms of non-volatile memory,including by way of example semiconductor memory devices. Theinformation carriers can, for example, be EPROM, EEPROM, flash memorydevices, magnetic disks, internal hard disks, removable disks,magneto-optical disks, CD-ROM, and/or DVD-ROM disks. The processor andthe memory can be supplemented by, and/or incorporated in specialpurpose logic circuitry.

To provide for interaction with a user, the above described techniquescan be implemented on a computer having a display device. The displaydevice can, for example, be a cathode ray tube (CRT) and/or a liquidcrystal display (LCD) monitor. The interaction with a user can, forexample, be a display of information to the user and a keyboard and apointing device (e.g., a mouse or a trackball) by which the user canprovide input to the computer (e.g., interact with a user interfaceelement). Other kinds of devices can be used to provide for interactionwith a user. Other devices can, for example, be feedback provided to theuser in any form of sensory feedback (e.g., visual feedback, auditoryfeedback, or tactile feedback). Input from the user can, for example, bereceived in any form, including acoustic, speech, and/or tactile input.

The above described techniques can be implemented in a distributedcomputing system that includes a back-end component. The back-endcomponent can, for example, be a data server, a middleware component,and/or an application server. The above described techniques can beimplemented in a distributing computing system that includes a front-endcomponent. The front-end component can, for example, be a clientcomputer having a graphical user interface, a Web browser through whicha user can interact with an example implementation, and/or othergraphical user interfaces for a transmitting device. The components ofthe system can be interconnected by any form or medium of digital datacommunication (e.g., a communication network). Examples of communicationnetworks include a local area network (LAN), a wide area network (WAN),the Internet, wired networks, and/or wireless networks.

The system can include clients and servers. A client and a server aregenerally remote from each other and typically interact through acommunication network. The relationship of client and server arises byvirtue of computer programs running on the respective computers andhaving a client-server relationship to each other.

Packet-based networks can include, for example, the Internet, a carrierinternet protocol (IP) network (e.g., local area network (LAN), widearea network (WAN), campus area network (CAN), metropolitan area network(MAN), home area network (HAN)), a private IP network, an IP privatebranch exchange (IPBX), a wireless network (e.g., radio access network(RAN), 802.11 network, 802.16 network, general packet radio service(GPRS) network, HiperLAN), and/or other packet-based networks.Circuit-based networks can include, for example, the public switchedtelephone network (PSTN), a private branch exchange (PBX), a wirelessnetwork (e.g., RAN, bluetooth, code-division multiple access (CDMA)network, time division multiple access (TDMA) network, global system formobile communications (GSM) network), and/or other circuit-basednetworks.

The transmitting device can include, for example, a computer, a computerwith a browser device, a telephone, an IP phone, a mobile device (e.g.,cellular phone, personal digital assistant (PDA) device, laptopcomputer, electronic mail device), and/or other communication devices.The browser device includes, for example, a computer (e.g., desktopcomputer, laptop computer) with a world wide web browser (e.g.,Microsoft® Internet Explorer® available from Microsoft Corporation,Mozilla® Firefox available from Mozilla Corporation). The mobilecomputing device includes, for example, a personal digital assistant(PDA).

Comprise, include, and/or plural forms of each are open ended andinclude the listed parts and can include additional parts that are notlisted. And/or is open ended and includes one or more of the listedparts and combinations of the listed parts.

One skilled in the art will realize the invention may be embodied inother specific forms without departing from the spirit or essentialcharacteristics thereof. The foregoing embodiments are therefore to beconsidered in all respects illustrative rather than limiting of theinvention described herein. Scope of the invention is thus indicated bythe appended claims, rather than by the foregoing description, and allchanges that come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

What is claimed is:
 1. A method of initializing or resetting a trackingelement, comprising: initializing or resetting, by an inertial trackingsystem, an initial pose of a first inertial measurement unit (IMU) whilethe first IMU is temporarily mechanically coupled to a first portion ofa linkage arm in a first reproducible reference pose, wherein theinitial pose of the first IMU is known and achieved when the first IMUis coupled to the linkage arm in the first reproducible reference pose;determining, by the inertial tracking system, a tracked pose of thefirst IMU, wherein the first IMU is mounted to a first object; andresetting the initial pose of the first IMU, thereby eliminating driftcaused over time between the tracked pose and the actual pose of thefirst IMU, when the first IMU is returned to the first reproduciblereference pose.
 2. The method of claim 1, further comprising:initializing or resetting, by the inertial tracking system, an initialpose of a second IMU while the second IMU is temporarily mechanicallycoupled to a second portion of the linkage arm in a second reproduciblereference pose, wherein the initial pose of the second IMU is known andachieved when the second IMU is coupled to the linkage arm in the secondreproducible reference pose; determining, by the inertial trackingsystem, a tracked pose of the second IMU relative to the first IMUwherein the second IMU is mounted to a second object; and resetting theinitial pose of the second IMU, thereby eliminating drift caused overtime between the tracked pose and the actual pose of the second IMU,when the second IMU is returned to the second reproducible referencepose.
 3. The method of claim 2, wherein the second IMU comprises a thirdkinematic coupling, and the second portion of the linkage arm comprisesa fourth kinematic coupling.
 4. The method of claim 3, wherein the thirdkinematic coupling is configured to kinematically couple to the fourthkinematic coupling to achieve the second reference pose, and furthercomprising: initializing or resetting the tracked pose of the second IMUoccurs when the fourth kinematic coupling is kinematically coupled tothe third kinematic coupling.
 5. The method of claim 2, furthercomprising: determining, by the inertial tracking system, that an errorassociated with the first IMU, the second IMU, or both, exceeds apredetermined threshold; and initializing or resetting the tracked poseof the first IMU, the tracked pose of the second IMU, or both, based onthe reference pose.
 6. The method of claim 2, wherein the step ofinitializing or resetting, by the inertial tracking system, the initialpose of the first IMU, the second IMU, or both is automaticallyinitiated by detecting when the first IMU is coupled to the linkage armin the first reference pose, the second IMU is coupled to the linkagearm in the second reference pose, respectively, or both.
 7. The methodof claim 2, further comprising: calculating a transform between thefirst IMU and the first object; and calculating a transform between thesecond IMU and the second object.
 8. The method of claim 2, furthercomprising: using the tracked poses to track the first object coupled tothe first IMU and the second object coupled to the second IMU as thefirst and second objects are moved independently.
 9. The method of claim1, wherein the linkage arm comprises a plurality of links and aplurality of joints between the links, and wherein each of the pluralityof joints comprises a position sensor to track a pose of the linkagearm.
 10. The method of claim 9, further comprising determining, usingdata from the position sensors, a pose of the first portion of thelinkage arm relative to the second portion of the linkage arm.
 11. Themethod of claim 9, wherein the position sensor is a joint encoder. 12.The method of claim 1, wherein the first IMU comprises a first kinematiccoupling and the first portion of the linkage arm comprises a secondkinematic coupling.
 13. The method of claim 12, wherein the firstkinematic coupling is configured to kinematically couple to the secondkinematic coupling to achieve the first reference pose, and furthercomprising: initializing or resetting the tracked pose of the first IMUoccurs when the second kinematic coupling is kinematically coupled tothe first kinematic coupling.
 14. The method of claim 1, wherein asecond portion of the linkage arm is coupled to a stationary object. 15.A method of initializing or resetting a tracking element, comprising:coupling a linkage arm with a first IMU and a second IMU in areproducible reference pose, wherein the actual pose of the first IMUand the second IMU are known when in the reproducible reference pose;setting an initial tracked pose of the first IMU and the second IMU whenthe linkage arm and the IMUs are in the reproducible reference pose,such that the tracked poses of the IMUs correspond with the actualposes; tracking the first IMU and the second IMU based on the initialtracked poses; and returning the first IMU and the second IMU to thereproducible reference pose with the linkage arm to eliminate draftoccurring between the actual poses and the tracked poses since theprevious setting of the initial tracked pose in the reproduciblereference pose.
 16. The method of claim 15, wherein the linkage armcomprises a plurality of links and a plurality of joints between thelinks, and wherein each of the plurality of joints comprises a positionsensor to track a pose of the linkage arm.
 17. The method of claim 16,further comprising determining, using data from the position sensors, apose of the first portion of the linkage arm relative to the secondportion of the linkage arm.
 18. The method of claim 16, wherein theposition sensor is a joint encoder.
 19. The method of claim 15, whereinthe first IMU comprises a first repeatable coupling mechanism andwherein the second IMU comprises a second repeatable coupling mechanism;wherein the first repeatable coupling mechanism and the secondrepeatable coupling mechanism are configured to engage with respectivefirst and second repeatable coupling mechanisms on the linkage arm; andwherein coupling a linkage arm with a first IMU and a second IMU in areproducible reference pose comprises engaging the first and secondrepeatable coupling mechanisms with the repeatable coupling mechanismson the linkage arm.
 20. A computer program product, tangibly embodied ina computer readable storage medium, the computer program productincluding instructions being operable to cause a data processingapparatus to: initialize or reset, by an inertial tracking system, aninitial pose of a first inertial measurement unit (IMU) and a second IMUwhile the first and second IMUs are temporarily mechanically coupledtogether by a linkage arm in a reproducible reference pose, wherein theinitial poses of the first IMU and second IMU are known and achievedwhen the IMUs are coupled together by the linkage arm in thereproducible reference pose; determine, by an inertial tracking system,a tracked pose of the first inertial measurement unit (IMU), wherein thefirst IMU is mounted to a first object; determine, by an inertialtracking system, a tracked pose of the second (IMU) relative to thefirst IMU wherein the second IMU is mounted to a second object; andreset the initial pose of the first IMU and the second IMU, therebyeliminating drift caused over time between the tracked pose and theactual pose of the first IMU and second IMU, when the linkage arm isreturned to the reproducible reference pose with the first IMU and thesecond IMU.